1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image using an image forming unit or a process cartridge which can be attached to and detached from the image forming apparatus.
2. Description of Related Art
A conventional image forming apparatus using an electrophotographic image forming process employs a process cartridge system in which a photosensitive drum and process means applied to it are integrally formed into a cartridge, and the cartridge (image forming unit) is detachably attachable to an image forming apparatus main body. According to this process cartridge system, a user can perform the maintenance operation of the apparatus without asking a service man to do it and thus, the operability can remarkably be enhanced. Thus, this process cartridge system is widely used in image forming apparatuses.
When such a process cartridge is attached to the image forming apparatus main body, it is necessary to precisely maintain a positional relation between the process cartridge and the image forming apparatus main body while keeping the appropriate operating feeling when the attaching operation is completed.
As a configuration for positioning the process cartridge to the image forming apparatus and holding the same, one using a wire spring as described in Japanese Patent Application Laid-open No. 11-174940 is widely used. According to this configuration, projections are provided on both sides of the process cartridge in its longitudinal direction, and the projections are engaged with and positioned to U-grooves formed in the cartridge attaching portions of the apparatus main body when the process cartridge is inserted along the guide section of the apparatus main body. When the cartridge projections are engaged with the U-grooves, the wire spring provided in the U-groove clamps the cartridge projection, thereby fixing the cartridge projection.
According to the configuration for fixing the cartridge projection using the wire spring, the clamping force of the wire spring with respect to the process cartridge projection becomes a resistance when the cartridge is inserted. Further, the projection and the wire spring are always in contact with each other until the projection abuts against a bottom of the U-groove. Thus, this resistance acts until the process cartridge is positioned at a normal position. As a result, when the insertion becomes insufficient, the cartridge may float or ride, which prevents the cartridge from being positioned at the normal position in some cases.
As a method for solving this problem, there is proposed a configuration using a toggle latch as positioning and fixing configurations of the process cartridge as described in Japanese Patent Application Laid-open No. 2006-98492.
FIG. 14 is a diagram for describing positioning and fixing configurations of the process cartridge according to the Japanese Patent Application Laid-open No. 2006-98492. In FIG. 14, a toggle arm 90 can turn between two positions, i.e., a first position (a position where the cartridge is opened) shown with a solid line, and a second position (a position where the cartridge is fixed) which is turned from the first position in the direction of arrow Y and which is shown with a broken line. The toggle arm 90 can turn between the first position and the second position by the turning shaft C.
As shown in FIG. 14, the toggle arm 90 is provided with an extension spring 91. The extension spring 91 is engaged with a spring fulcrum D provided on a side plate 92, and a spring hook 93 provided on the toggle arm 90. When the toggle arm 90 is in the first position shown with the solid line in FIG. 14, the spring hook 93 is located on one side (upper side in FIG. 14) of a line segment C-D connecting the spring fulcrum D on the side plate and the turning shaft C. When the toggle arm 90 is located in the second position shown with the broken line, the spring hook 93 is located on the other side (lower side in FIG. 14) of the line segment C-D.
If the above configuration is employed, when the toggle arm 90 is in the first position shown with the solid line, a moment in the X direction acts due to the extension spring 91. Therefore, no other external force acts, and the toggle arm 90 is held in the first position. When the cartridge is inserted and a projection 94 provided on the cartridge is engaged with a U-groove 95 formed in the apparatus main body from this state, the projection 94 pushes an engaging section 96 of the toggle arm 90. With this, a moment in the direction of the arrow Y in FIG. 14 is given to the toggle arm 90, and the toggle arm 90 turns in the direction of the arrow Y against the extension spring 91. When the spring hook 93 moves to a position lower than an extension of the line segment C-D, a moment generated by the extension spring 91 is changed from the X direction to the Y direction, and when no other external force acts, it is held in the second position.
With this, when the cartridge is positioned, the resistance of the spring does not act until the positioning operation is completed, and the operability is remarkably enhanced. After the toggle arm 90 moves to the second position, the projection 94 is pushed against the U-groove 95 by a biasing force of the extension spring 91. Thus, the cartridge is reliably fixed at the positioned location.
In Japanese Patent Application No. 2006-98492, a spring force of the toggle arm 90 presses the process cartridge to prevent the process cartridge from floating, and this leads to stabilization of an image quality. Thus, a spring 91 used for the toggle arm 90 is set to be strong.
However, if the spring 91 is made strong and the movement of the toggle arm 90 to the second position is delayed, the rotation force produced by the spring force becomes a maximum at both ends of turning motion of the toggle arm 90 when the cartridge is attached or detached, and this provides a heavy feeling when the process cartridge is attached or detached. If this operating feeling is too heavy, a user may falsely believe that the process cartridge has been successfully attached even through the attaching operation has not yet been completed.
Therefore, if a dead point near a neutral point of the extension spring 91 is set early, such a heavy operating feeling can be lightened. That is, if it is pushed a little, the toggle arm 90, which is a fixing member is immediately operated.
However, the above setting causes the following problem. That is, the toggle mechanism is operated even if a user accidentally moves the toggle arm 90. If it is once operated, a locked state is established and thus, the guide portion of the housing is occluded. That is, the toggle arm 90 moves faster than the process cartridge, and this prevents the process cartridge from being inserting.
Especially when the process cartridge is large and can handle A3 paper, a user can not sufficiently visually check both positions in the longitudinal direction and thus, the toggle arm 90 moves accidentally every so often. That is, a user is forced to roughly guide one side and then insert the other side in the longitudinal direction, and attach them to the pair of U-grooves 95 in a straight manner while paying attention to keep the cartridge parallel as precisely as possible. At that time, the toggle arm 90, which is a projection located near an inlet of the U-groove 95, is easily caught on an end of the thin and long process cartridge in its longitudinal direction, since it is designed such that it is moved lightly because emphasis is placed on producing a light operating feeling, thus, there is a problem that it is easily operated.
If the toggle arm 90 is operated accidentally, it is required to return the toggle arm 90 to the first position again before the process cartridge is inserted and thus, there is a problem that usability deteriorates.